In recent years, the technology related to a wavelength division multiplexing (hereinafter, “WDM”) system is being developed. In the WDM system, plural light signals that have varying wavelengths are multiplexed and transmitted through a single optical fiber as if the light signals are transmitted through separate optical fibers. In the WDM system, the transmission capacity can be increased by, for instance, reserving a large band of many gigabit (hereinafter, “Gbit”) per wavelength, multiplexing and transmitting the wavelengths of several hundreds of Gbit over a single optical fiber, and increasing the number of multiplexed wavelengths.
In the WDM system, in order to densely multiplex the light signals of varying wavelength bands, it is necessary to narrow down (to, for example, 50 giga hertz) the interval between each wavelength band, avoid overlapping of the wavelengths, and stabilize each wavelength band with precision. An optical module, which is employed as an optical transmission device compatible to the WDM system, outputs from the optical fiber the light signals, which have a specific wavelength band and are output from a mounted laser diode (hereinafter, “LD device”), after stabilizing the oscillation wavelength of the LD device. In this case, the light signals, which are output from all the optical fibers of the plural optical modules that have varying wavelength bands, are input into a single optical fiber via an optical coupler and then the wavelengths are multiplexed.
Conventional Art 1
A conventional optical module of this kind is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2001-291928. This optical module monitors the oscillation wavelength of an LD device, adjusts the temperature of the LD device to lock the wavelength. This disclosure also discloses an optical module that is formed by serially placing an LD device which outputs as light signals laser light in forward as well as backward directions, a first photodiode (hereinafter, “PD device”) that has a semi-transparent structure and receives as well as transmits the rear-facet light signals from the LD device, a wavelength filter that is formed of an etalon filter on which the light signals transmitted from the first PD device are incident and which possesses the wavelength-dependent transmission characteristics of those light signals, and a second PD device that receives the light signals transmitted from the wavelength filter. The output current of the first PD device is divided by the output current of the second PD device and, using the feature that the result of this division changes according to the wavelength, the oscillation wavelength of the LD device is monitored and according to the monitoring result the temperature in the vicinity of the LD device is controlled. Moreover, the output current of the first PD device is monitored based on the intensity of the output light from the LD device.
However, as this kind of an optical module employs in the first PD device a PD device that has a semi-transparent structure and inputs in succession into the wavelength filter and the second PD device the transmitted light from the first PD device, the amount of light of the light signals, which are transmitted by the wavelength filter and received by the second PD device, reduces because of transmission through the first PD device. As a result, the S/N characteristic of the signals deteriorates, and the oscillation wavelength of the LD device can not be monitored as desired.
Conventional Art 2
The optical module according to the conventional art 1 includes a beam splitter that splits into two a rear-facet laser light output from the LD device, a first PD device that receives one part of the light signals split by the beam splitter, a wavelength filter through which the remaining part of the light signals split by the beam splitter pass, and a second PD device that receives the light signals that have passed through the wavelength filter. The wavelength oscillation of the LD device is monitored based on the output current of the first PD device and the output current of the second PD device, and the intensity of the output light from the LD device is monitored according to the first PD device. A technology identical to this is also disclosed in the Japanese Patent Laid-Open Publication No. 2000-56185 and the Japanese Patent Laid-Open Publication No. 2001-244557.
The conventional technologies described in either the conventional arts 1 or 2 do not disclose a concrete structure of an optical component employed as the wavelength filter. Moreover, there is no mention either about the problems caused in the optical component due to a change in the temperature environment when the optical component is mounted inside the optical module or about the structure required to hold plural optical components. Although the Japanese Patent Laid-Open Publication No. 2000-56185 teaches to fix the wavelength filter onto the board either by soldering or with an adhesive, there is no disclosure of a concrete support structure to fix the wavelength filter by considering the thickness of the board, while holding the wavelength filter upright so that the laser light output from the LD device and parallel to the board passes through the wavelength filter.
Conventional Art 3
In the Japanese Patent Laid-Open Publication No. 2001-244557, a wavelength filter is fabricated as an optical component by juxtaposing two optical components, namely, a complex-refractive-index crystal that includes an etalon crystal structure and a polarizer. A technology is described that monitors the oscillation wavelength of a LD device by receiving into plural receiving devices the light passing through the wavelength filter. Especially in FIG. 8 of the disclosure, an example of a wavelength filter that cancels out the change in the refractive indices of the crystals corresponding to the crystalline temperature is disclosed, wherein two complex-refractive-index crystals are mounted beside each other so that according to the increase in the crystalline temperature the refractive index of one crystal increases and the refractive index of the other crystal decreases. An example is disclosed in which a combination of a YVO4 crystal and a β-BaB2O4 crystal is employed as the two complex-refractive-index crystals.
However, the thickness of the complex-refractive-index crystal is about 0.05 millimeter (mm), making the crystal very thin and fragile. Hence it is difficult to mount the complex-refractive-index crystal inside the optical module. Also, there is no disclosure either of a concrete structure for fixing plural wavelength filters inside the optical module so that the laser light output from the LD device passes through the wavelength filters or of the problems arising due to fixing of the wavelength filters.
In FIG. 8 of Japanese Patent Laid-Open Publication No. 2001-244557, a technology identical to the conventional art 2 is described that monitors wavelengths by receiving into two PD devices the signals that are split into two by a beam splitter.
As part of mounting the thin complex-refractive-index crystal inside the optical module, the inventors of the present invention conducted experiments. The results of these experiments are explained below.
To hold a complex-refractive-index crystal that is 1 mm or less thin upright inside the optical module, it is necessary to place a metal holder inside the optical module and hold the incident surface or the output surface of the laser light in the complex-refractive-index crystal by bonding the complex-refractive-index crystal to the metal holder.
However, if the complex-refractive-index crystal is soldered to the metal holder, a heat stress is generated at the junction because of the difference between coefficients of linear expansion of the metal holder and the complex-refractive-index crystal, and the optical characteristics of the complex-refractive-index crystal deteriorate due to the strain caused by a residual stress. Further, due to the change in the stress caused by a soldering-related shrinkage, cracks appear in the region around the junction and damage the complex-refractive-index crystal. This damage occurs even if the coefficients of linear expansion of the metal holder and the complex-refractive-index crystal are almost equal.
In the optical module according to the conventional art 1, the first PD device and the second PD device are placed before and after the wavelength filters that are serially placed over the optical path of the LD device. This makes the PD device longer in the arranging direction and hinders the downsizing of the optical module.
In the optical module according to the conventional art 2, the light signals, which are split into two by the beam splitter on a plane parallel to the bottom surface of the optical module, are received by the first PD device and the second PD device, respectively, that are disposed at separate positions on the sides of the output direction of the LD device. The LD device is also disposed on the plane parallel to the bottom surface of the optical module. Consequently, the area on the plane parallel to the bottom surface of the optical module that is occupied by the first PD device and the second PD device increases. This again hinders the downsizing of the optical module.
Also, it is necessary to set the first PD device and the second PD device adequately apart from each other and adjust the alignment of optical axes of the first PD device and the second PD device so that the directions of the optical axes split by the beam splitter coincide with the respective orthogonal directions. When each optical component is fixed inside the optical module, this adjustment can be very cumbersome.
The conventional art 3 has identical problems.
Moreover, in any of the conventional arts 1 through 3, there is no disclosure of a concrete support structure in order to hold the wavelength filters, which are formed by plural optical components, upright on the board.
Hence, the inventors of the present invention studied a structure in which the wavelength filters are bonded to the metal holder and the metal holder is disposed inside the optical module in such a way that the wavelength filters receive the rear-facet output light of the LD device. The experiments carried out by the inventors showed that the cracks appear in the complex-refractive-index crystal in the region where it is bonded to the metal holder.
When the inventors looked for the reason behind the problem, it became evident that although the coefficients of linear expansion of the metal holder and the complex-refractive-index crystal are almost the same, the coefficient of linear expansion of the complex-refractive-index crystal is larger in a specific direction (optical axis direction) owing to the anisotropic nature of the complex-refractive-index crystal. This difference in the coefficients of linear expansion is the cause of the heat stress generated at the junction.
Further, it was also discovered that when the complex-refractive-index crystal, which includes a cut edge, is bonded to the metal holder, the heat stress acts on the fine cracks or chaps on the cut edge, widening the cracks or the chaps. Consequently, the complex-refractive-index crystal is damaged resulting in deterioration of the optical characteristics of the complex-refractive-index crystal.